Reid has created a world in which angels and humans exist in tandem. In fact, what makes Reid's mythology unique is that some human souls are brought forth from their angel counterparts.
Alex and Haley are interesting characters. Their relationship is believable and well thought out.
The only negative I can bring up is that the omniscient POV wasn't very clear to me and I found myself easily confused at different points in the story. But, this might just be the way I read it.